(1) Field of the Invention
Generally, this invention pertains to a circuit for the generation of AC ringing voltages for use in conjunction with a central processing unit of a telephone central office and more particularly pertains to a digitally activated ring generation circuit for approximating the sinusoidal ringing voltage by incremental steps and applying the voltage thereby generated to the subscriber line.
(2) Description of the Prior Art
Typically, ringing generators are implemented via a collection of analog circuit equipment. With the advent of computer controlled electronic switching systems, a need arose for simple interfacing between the computer control and the ringing generator circuit. Usually elaborate interface connections had to be designed for interfacing the computer controlled logic with the logic of the ringing circuit. The logic of computers is digital in nature and of relatively low power, whereas the logic of ringing circuits is analog in nature and the ringing voltages themselves are of a relatively high level.
Such analog interface circuits as those mentioned above are costly, complex in design and require that high voltages be distributed throughout the ringing circuitry of a telephone central office.
Typically, the hazardous high voltages were developed at one particular point and bused out to the various circuits requiring this voltage. Consequently, protection was required for the AC converters. Due to the busing arrangement and the number of terminals required to connect these high voltages, a considerable amount of space was consumed by the resultant designs.
Typical analog ringing generator circuits are shown by U.S. Pat. Nos. 4,015,091, issued on Mar. 29, 1977, to T. N. Rao, et al and 4,025,729, issued on May 24, 1977 to D. E. Stone. Such circuits disclose ringing signal generators comprising direct current level converters having the ringing voltage level thereby generated interrupted at a particular rate.
U.S. Pat. No. 4,001,516, issued on Jan. 4, 1977 to G. Weisigk, et al teaches an apparatus for feeding ringing currents from a ringing current supply to particular line circuits. Such an elaborate design as taught by Weisigk et al is not required in the present invention due to the fact that each frequency is derived upon a single circuit card.
Lastly, U.S. Pat. No. 4,042,786, issued on Aug. 16, 1977, to L. Freimanis teaches a telephone ringer circuit. A high capacitance element is charged by DC voltage present on the telephone line and discharged across an electric mechanical ringer when a tone detector receives a tone alerting signal.
Therefore, it is the object of the present invention to provide a ringing circuit for use in conjunction with a central processing unit of a telephone central office which digitally interfaces with the central processing unit and manipulates the high voltages associated with ringing circuit in a safer, more efficient and less costly manner.
It is a feature of the present invention to provide for the generation of a plurality of distinct ringing frequencies and to provide as an option selection of a particular set of ringing frequencies for each of such ringing circuits that are provided. Flexibility is provided by information representing an approximation to each of the particular frequencies stored in a read only memory.
Further, it is a feature of the present invention to provide a quick response in order to remove the ring once the particular subscriber has answered the phone but not to respond so quickly that transient errors are handled as true ring trip.
In addition, it is the feature of the present invention to provide a ring generating circuit for use in conjunction with a coin collection/refund circuit, so that the two circuits are compatible and utilize common circuitry. Thereby, both the functions of ringing and coin collect can be provided on one circuit, thereby necessitating no further network connections for ringing and coin control operations. A circuit comprising both operations may be connected by the telephone switching center in one operation.
Also, it is the feature of the present invention to provide for the elimination of protectors for the high voltages which must be bused throughout the network in conventional ringing schemes.